Substantives that begin with a- or ha- are feminine but require the masculine article ‘el’ to respect the pronunciation. However in the plural form, the article respects the gender of the word.
_el agua (las aguas)
_el águila (las águilas)
_el hacha (las hachas)

Some words are ‘cut in half’ by their daily use. These are feminine even if they end in –o, because they end in -a in the original uncut form.
_la moto (motocicleta)
_la foto (fotografía)
­_la disco (discoteca)

Some substantives have different meanings depending on their gender.
_la policía (the police) _el policía (the policeman)
_la guía (the book guide) _el guía (the guide (person))